The purpose of this study was to evaluate the effects of physical activity on the body composition of ageing female rats. Female pathogen-free Long-Evans rats were housed either in individual 7 x 14 x 8 inch cages or in cages with attached running wheels to which they had free access. The runners ate significantly more than the sedentary rats. Food intake from month 10 through month 24 of age averaged 14.6 +/- 0.7 g for the sedentary group, and 18.3 +/- 2.2 g for the active group. The body fat content of the sedentary rats was approximately 50% higher, while their lean body mass and protein content were significantly lower than that of the runners at age 25 months. Total body weight was similar in the active and sedentary groups. Percent body fat and protein of the 25-month-old physically active rats were not significantly different from that of 9-month-old rats, while the sedentary 25-month-old rats had a significantly higher body fat content and a lower body protein content than the 9-month-old animals. These results suggest the possibility that the changes in body composition that occur during middle age in sedentary female rats are largely due to physical inactivity, and that the lean tissue wasting that occurs as the result of the aging process is a late event that occurs closer to the end of life.